This invention relates to the monitoring of cardiac output, which can be critical in a post-cardiac surgery patient. Detection, diagnosis, and treatment of volume depletion, tamponade, and ventricular failure can be life-saving.
The measurement of cardiac output can be critical to patient care. 500,000 patients undergo cardiac surgery each year, and many of these patients receive pulmonary artery catheters to aid in hemodynamic management. There have been numerous efforts to replace the pulmonary artery catheter as the gold standard for measurements of cardiac output. Most of these techniques are inaccurate, expensive, or unreliable.
One of these, the Thoracic Electrical Bioimpedance (TEB) has never caught on because of a poor signal-to-noise ratio, low accuracy, and poor reliability. The electrical signal used for TEB is produced by variations in the volume of conductor in the chest. Unfortunately, measurement of the impedance of the chest relies on an accurate knowledge of the current path and associated electrical impedance of structures between the electrodes and the structure changing in volume. Fluid content of the lungs will effect the current path and consequently the magnitude of measured voltages. Cardiac pathology can affect right and left ventricular volumes and produce inaccuracies in the measurement.
Traditionally, TEB signals were thought to derive solely from changes in conductor volume. However, changes in the flow of conducting fluids with no change in conductor volume can effect impedance of that fluid. Flow measurements based on a change in conductor volume are critically dependent on mechanical distensibility of the structure containing the flow. If the mechanical distensibility is nonlinear (as in the aorta or heart) with pressure changes, the accuracy of the flow measurement will suffer over a wide range of pressures and flows. If the flow measurement is based on a true change in flow, the mechanical distensibility of the structure does not matter.
External thoracic electrodes for the measurement of thoracic impedance have been used for years but are not accurate or reliable. Intravascular recordings of left ventricular volume have been made but are highly invasive and only appropriate for animal or small clinical studies. Esophageal measurements have been made but had not been sufficiently accurate.
Endotracheal measurements were hypothesized by Ascher Schmulowitz, U.S. Pat. Nos. 5,782,774 and 5,791,349, the contents of which are incorporated by reference herein. This technology was further described in Wallace et al, Endotrachael Cardiac Output Monitor. Anesthesiology, Vol 92 No 1, p178-189, January 2000. The content of that article is also incorporated by reference herein. The Endotracheal Cardiac Output Monitor is termed the ECOM.
At the present time, the pulmonary artery catheter is the most commonly used reliable device for the measurement of cardiac output. Placement of a pulmonary artery catheter is expensive and has associated risk including: death; infection; hemorrhage; arrhythmias; carotid artery; thoracic duct, vena caval, tracheal, right atrial, right ventricular, mitral and tricuspid valvular, and pulmonary artery injury. Little evidence suggests that placement of a pulmonary artery catheter improves survival, and several suggest an increase in morbidity and mortality.
A simple, inexpensive, reliable method for the measurement of cardiac output without the associated risks and costs of a pulmonary artery catheter would be useful.
An object of the invention is to provide a method and system for the measurement of cardiac output which minimizes the disadvantages of prior art techniques.
According to the invention, there is provided an internal cardiac output monitor system, apparatus, and method (ICOM) for use in cardiac surgery patients who have temporary cardiac pacing in place, namely the pacing electrodes. Two extra wires or leads, namely first and second electrodes, are placed in relationship in the peri-aortic adventitia or fat pad. An alternating electrical current (100 kHz 2 ma) is injected from the atrial pacing wires or leads, namely the third electrode to a ground electrode (fourth). Impedance measurements are taken directly from the aorta. These measurements have a signal-to-noise ratio that is more accurate than extra-thoracic measurements. The current path is limited and relatively insensitive to changes in lung water. The changes in impedance caused by flow can be measured essentially directly. The voltage changes are induced primarily by blood flow dynamics since the current conducted between electrodes flows primarily through high-conductivity blood.
When the temporary pacing wires or leads are removed on postoperative day 4 or 5, the two extra wires or leads for the ICOM are removed at the same time. This system allows inexpensive, reliable, accurate measurement of cardiac output in postoperative cardiac patients for 4 to 5 days with minimal risk. It takes less time to place, is safer, and is less expensive than a pulmonary artery catheter.
The ICOM system is an inexpensive, accurate, reliable system to directly measure aortic impedance and derive cardiac output measurements in post-cardiac surgery patients. The placement of the two extra wires or leads in the peri-aortic adventitia or fat pad provides for accurate functioning. These wires should be separated by at least 10 mm. Both or either orthogonal-to-flow and/or parallel-to-flow placement can be used. Placement orthogonal-to-flow is superior. The impedance measurement circuit should be able to measure a delta Z signal with about 1% accuracy with a Zo of 1 ohm.
The ICOM technique controls the injected current path and measures the changes in impedance of a simple structure to produce a more reliable cardiac output measurement. This technique which increases the signal-to-noise ratio by 100 reduces the electronics cost and makes the technique more commercially viable.
The foregoing and other objects, features, and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments which makes reference to the figures.